This invention relates generally to methods and apparatus for use in bag-making machines for manufacturing bags of thermoplastic material and, more particularly, to such methods and apparatus adapted for the manufacture of bags which are formed from tubular webs of thermoplastic material with the completed bag structure having a thermally welded seal and an open top. Still more particularly, the present invention relates to such methods and apparatus whereby any portion of the thermoplastic web which may have become bonded during the sealing operation to the apparatus is stripped therefrom to avoid the possibility of the web adhering to the machine during operation.
Various techniques have been proposed for bottom seal bag-making machines which generally utilize means for intermittently advancing a web of thermoplastic material of tubular, flexible stock and wherein while the web is at rest between intermittent advancements thereof, the web is held under tension whereupon a blade is reciprocated to cut the web and a heated sealing bar then reciprocated to seal the plies of the web at a portion which overlies a sealing pad to thereby form the bottom-seal of the bag.
It has been recognized that during the sealing operation described above, it is not uncommon for the sealed portion of the web to become bonded to the sealing pad of the machine at least to some extent. Such bonding or adherence of the bottom-seal to the sealing pad has severely inhibited the smooth operation of the machine in that when the draw rollers are actuated to further advance the web into position for the next cutting and sealing operations, the bottom-seal so formed constitutes the leading edge of the web being advanced and should the same have become bonded to the sealing pad, the web will not advance but, rather will become bunched up in the region of the sealing pad thereby requiring the machine to be shut down to remedy the situation. Of course, such periods of down time for these high-speed machines are quite costly.
Several attempts have been made to overcome the problems described above. For example, in machines exemplified by those disclosed in U.S. Pat. No. 4,101,369 assigned to G. T. Schjeldahl Company and 4,105,489 assigned to F. M. C. Corporation, methods and apparatus are provided wherein subsequent to the web being transversely severed and the seal bars sealing the web along a margin which defines the bottom of the bag, the web is momentarily fed rearwardly or retracted to strip any portion of the web which may have become bonded to the seal bars during the sealing operation.
However, such techniques which incorporate a reversal in the direction of feed of the web are not entirely satisfactory. Thus, such arrangements tend to result in a weak seal being formed and, in some cases, the bottom-seal is torn during the reversal of the direction of the web. Further, the skirt portion of the web is often curied by this action. Since the reversal of the direction of feed of the web incorporated in such techniques generally require the use of a clutch which reverses the direction of the draw rolls and associated elements of the machine, undue stresses are exerted on the mechanical train of the bag-making machine which contribute to early failure thereof.
Other arrangements have been suggested to overcome the problem of the bottom-seal adhering or bonding to the sealing structure of the bag-making machine. For example, U.S. Pat. No. 4,131,503 discloses a technique for cutting and sealing thermoplastic material wherein the thermoplastic web is advanced over a sealing roll having a Teflon covering whereupon the web, under tension, is severed and the bottom-seal formed through movement of a sealing bar against the web which overlies the sealing roll. However, this arrangement has not overcome the problems described above since the bottom-seal formed during the sealing operation still tends to bond or adhere to the sealing roll to inhibit the further advancement of the web.